Optical transmitting and receiving system

ABSTRACT

A method and apparatus for transmitting and receiving optical signals in a bi-directional transmission system ( 1 ) using a common transmission medium ( 3 ). Laser radiation at the first wave length and intensity is directed from a laser source ( 4 ) to a non-linear optical device ( 6 ). The non-linear optical device ( 6 ) is responsive to radiation at the first wavelength and intensity is directed from a laser source ( 4 ) to non-linear optical device ( 6 ). The non-linear optical device ( 6 ) is responsive to radiation at the first wavelength and intensity to emit radiation of a second wavelength which is transmitted through the medium ( 3 ). The non-linear optical device ( 6 ) transmits incident radiation of the second wavelength have less than a selected intensity. A selective mirror ( 5 ) is interposed between the source ( 4 ) and non-linear optical device  6  to pass radiation of the first wavelength from the source ( 4 ) and directed towards a target ( 7 ) radiation of a second wavelength passing from the medium through the non-linear optical device ( 6 ) in the opposite direction to radiation from the source ( 4 ).

FIELD OF THE INVENTION

This invention relates to an optical transmitting and receiving systemfor use in bi-directional transmission in a common transmission medium.In the present context, a common transmission medium refers to thecommon volume of any suitable medium, such as air, space or an opticalfibre which is used to transmit optical signals in opposite directionsat the same time.

BACKGROUND ART

In many optical applications, it is desirable to utilise the sameoptical path for signals travelling in opposite directions. Theadvantages that flow from this configuration include a reduction in theamount of optical alignment required, and where the path is through adedicated medium such as an optical fibre, a reduction in the amount ofmedium required.

Prior attempts to achieve bi-directional transmission of opticalsignals, have utilised laser sources operating through partiallyreflective mirrors or, in the case of transmission through opticalfibres, directional couplers. In both cases, the losses incurred by theuse of the partially reflective mirrors and the directional couplersrespectively have lowered the efficiency of operation.

DISCLOSURE OF THE INVENTION

It is an object of this invention to provide an improved opticaltransmitting and receiving system and an improved method forbi-directional transmission of optical signals in a common transmissionmedium.

Accordingly, the present invention provides an optical transmitting andreceiving system for a bi-directional transmission system using a commontransmission medium, said transmitting and receiving system including asource of a laser radiation of a first wavelength and intensity,non-linear means responsive to radiation at said first wavelength andintensity to emit radiation of a second wavelength, said non-linearmeans transmitting incident radiation of said second wavelength havingless than a selected intensity, selective reflecting means interposedbetween said source and said non-linear means to pass radiation of saidfirst wavelength from said source and reflect toward a target radiationof said second wavelength passing through the non-linear means andtravelling in the opposite direction to radiation from said source.

Preferably, the non-linear means is an optical parametric oscillator(OPO). The active material of such an oscillator can be chosen to have athreshold intensity for incoming radiation below which conversion to theother wavelength will essentially not occur.

The optical transmitting and receiving system of this invention hasparticular application to fibre optic communications systems. Theradiation generated by the non-linear means at the second frequency islaunched into an optical fibre. At the other end of a typical fibre, thesignal will be much weaker and the intensity will be less than thechosen operating threshold of the non-linear device. The signal willthus pass through the non-linear device and be reflected to thedetector.

Preferably, the selective reflecting means includes a dichroic mirrorwhich reflects radiation of the second wavelength to a target in theform of a suitable detector.

The optical transmitting and receiving system also preferably furtherincludes means to launch the radiation of said second wavelength emittedby the non-linear means for transmission through a medium to a likeoptical transmitting and receiving system, and means to direct anoptical signal from said like optical transmitting and receiving systemthrough said non-linear means to said reflecting means.

The medium used for transmission can be any suitable propagating mediumsuch as air, space or an optical fibre. Where required, adjustableattenuation can be provided for the radiation of said second wavelengthemitted from said non-linear means to allow the intensity level of theoptical signal reaching the like optical transmitting and receivingsystem to be reduced below said selected intensity.

Preferably, the laser source is a Nd:YAG laser operating at 1.06 μm. Thenon-linear means preferably converts the laser emission to 1.54 μm. Inthis form, the invention allows free space laser communications at theeye safe frequency of 1.54 μm, operating bi-directionally in the samespace. This has the significant advantage of avoiding the need foralignment of multiple optical apertures.

One embodiment of the invention will now be described, by way of exampleonly, with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing is a schematic illustration of a lasercommunication system incorporating the optical transmitting andreceiving system according to this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to the drawing, the laser communication system 1 comprises twooptical transmitting and receiving systems 2, which respectively launchsignals for transmission through a medium 3. Each of the systems 2comprises an Nd:YAG laser 4 appropriately pumped by a flash lamp orlaser diode in the conventional manner to produce a laser emission at awavelength of 1.06 μm. A dichroic mirror 5 is interposed between thelaser 4 and a non-linear optical device 6 in the form of an OPO. Thedichroic mirror 5 passes radiation at wavelengths around 1.06 μm, but ishighly reflective at wavelengths around 1.5 μm. The laser radiationemitted by laser 4 passes through dichroic mirror 5 to OPO 6. Theintensity of the incident laser radiation is above the thresholdintensity for the OPO, which converts the incident radiation toradiation at a wavelength of 1.5 μm for launching into the transmissionmedium 3. Any suitable form of optics can be used for launching theoptical signal for transmission according to the type of medium.

The signals transmitted through the medium by each of the transmittingand receiving systems 2 travel along the same optical path and are thusreceived at the other of the transmitting and receiving systems. If thelaunch intensity of the signal is appropriately chosen, the transmissionof the signal through the medium reduces the intensity of the radiationto below the operational threshold of the OPO. The incoming opticalsignal thus passes through the OPO substantially unchanged and isreflected by dichroic mirror 5 to a detector 7 in each case. It will beapparent that a practical laser transmission system would includesuitable modulating equipment associated with each laser 4 anddemodulating equipment associated with each detector 7.

What is claimed is:
 1. An optical transmitting and receiving system fora bi-directional transmission system using a common transmission medium,said transmitting and receiving system including: a source of a laserradiation of a first wavelength and a first intensity, a non-lineardevice responsive to radiation at said first wavelength and said firstintensity to emit radiation of a second wavelength, said non-lineardevice transmitting received radiation of said second wavelength havinga second intensity less than a predetermined intensity, and a selectivereflector interposed between said source and said non-linear device topass radiation of said first wavelength from said source and reflecttoward a detector, said received radiation of said second wavelengthpassing through the non-linear device and travelling in the oppositedirection to radiation from said source.
 2. An optical transmitting andreceiving system as claimed in claim 1 wherein said non-linear device isan optical parametric oscillator.
 3. An optical transmitting andreceiving system as claimed in claim 1 wherein said selective reflectoris a dichroic mirror.
 4. An optical transmitting and receiving system asclaimed in claim 1 wherein said common transmission medium is an opticalfibre.
 5. An optical transmitting and receiving system as claimed inclaim 1 further including a device to selectively attenuate theradiation of said second wavelength emitted by said non-linear device.6. An optical transmitting and receiving system as claimed in claim 1wherein said second wavelength is an eyesafe wavelength.
 7. An opticaltransmitting and receiving system as claimed in claim 6 wherein saidsecond wavelength is about 1.54 μm.
 8. An optical transmitting andreceiving system as claimed in claim 1 wherein said source is a Nd:YAGlaser operating at a wavelength of about 1.06 μm.
 9. A lasercommunications system including two optical transmitting and receivingsystems as claimed in claim
 1. 10. A method of transmitting andreceiving optical signals in a bi-directional transmission system usinga common transmission medium, said method including: directing laserradiation of a first wavelength and intensity from a source through anon-linear device responsive to radiation at said first wavelength andintensity to emit radiation of a second wavelength for transmissionthrough said medium, and directing received radiation of said secondwavelength from said medium through said non-linear device to selectivereflector interposed between said source and said non-linear device,said selective reflector passing radiation of said first wavelength fromsaid source and reflecting toward a detector, said received radiation ofa said second wavelength travelling in the opposite direction toradiation from said source.
 11. A method as claimed in claim 10 whereinsaid non-linear device is an optical parametric oscillator.
 12. A methodas claimed in claim 10 wherein said selective reflector is a dichroicmirror.
 13. A method as claimed in any one of claims 10 to 12 whereinsaid common transmission medium is an optical fibre.
 14. A method asclaimed in any one of claims 10 to 13 further including selectivelyattenuating the radiation of said second wavelength emitted by saidnon-linear device.
 15. A method as claimed in claim 10 wherein saidsecond wavelength is an eyesafe wavelength.
 16. A method as claimed inclaim 15 wherein said second wavelength is about 1.54 μm.
 17. A methodas claimed in claim 1 wherein said source is a Nd: YAG laser operatingat a wavelength of about 1.06 μm.